Acrolein (systematic name: propenal) is the simplest unsaturatedaldehyde. It is a colourless liquid with a piercing, disagreeable, acrid smell. The smell of burnt fat (as when cooking oil is heated to its smoke point) is caused by glycerol in the burning fat breaking down into acrolein. It is produced industrially from propylene and mainly used as a biocide and a building block to other chemical compounds, such as the amino acidmethionine.

About 500,000 tons of acrolein are produced in this way annually in North America, Europe, and Japan. Additionally, all acrylic acid is produced via the transient formation of acrolein. The main challenge is in fact the competing over oxidation to this acid. Propane represents a promising but challenging feedstock for the synthesis of acrolein (and acrylic acid).

When glycerol (also called glycerin) is heated to 280 °C, it decomposes into acrolein:

(CH2OH)2CHOH → CH2=CHCHO + 2 H2O

This route is attractive when glycerol is co-generated in the production of biodiesel from vegetable oils or animal fats. The dehydration of glycerol has been demonstrated but has not proven competitive with the route from petrochemicals.[5]

Acrolein is a relatively electrophilic compound and a reactive one, hence its high toxicity. It is a good Michael acceptor, hence its useful reaction with thiols. It forms acetals readily, a prominent one being the spirocycle derived from pentaerythritol, diallylidene pentaerythritol. Acrolein participates in many Diels-Alder reactions, even with itself. Via Diels-Alder reactions, it is a precursor to some commercial fragrances, including lyral, norbornene-2-carboxaldehyde, and myrac aldehyde.[4]

Acrolein is mainly used as a contact herbicide to control submersed and floating weeds, as well as algae, in irrigation canals. It is used at a level of 10 ppm in irrigation and recirculating waters. In the oil and gas industry, it is used as a biocide in drilling waters, as well as a scavenger for hydrogen sulfide and mercaptans.[4]

A number of useful compounds are made from acrolein, exploiting its bifunctionality. The amino acid methionine is produced by addition of methanethiol followed by the Strecker synthesis. Acrolein condenses with acetaldehyde and amines to give methylpyridines. It is also thought to be an intermediate in the Skraup synthesis of quinolines, but is rarely used as such due to its instability.

Acrolein will polymerize in the presence of oxygen and in water at concentrations above 22%. The color and texture of the polymer depends on the conditions. Over time, it will polymerize with itself to form a clear, yellow solid. In water, it will form a hard, porous plastic.

Connections exist between acrolein gas in the smoke from tobacco cigarettes and the risk of lung cancer.[10] In terms of the "noncarcinogenic health quotient" for components in cigarette smoke, acrolein dominates, contributing 40 times more than the next component, hydrogen cyanide.[11] The acrolein content in cigarette smoke depends on the type of cigarette and added glycerin making up up to 220 µg acrolein per cigarette (,[12][13]). Importantly, while the concentration of the constituents in mainstream smoke can be reduced by filters, this has no significant effect on the composition of the side-stream smoke where acrolein usually resides, and which is inhaled by passive smoking.[14][15]E-cigarettes, used normally, only generate "negligible" levels of acrolein (less than 10 mcg "per puff").[16][17]

Cyclophosphamide and Ifosfamide treatment results in the production of acrolein.[18] Acrolein produced during cyclophosphamide treatment collects in the urinary bladder and if untreated can cause hemorrhagic cystitis.

The "acrolein test" is for the presence of glycerin or fats. A sample is heated with potassium bisulfate, and acrolein is released if the test is positive. When a fat is heated strongly in the presence of a dehydrating agent such as potassium bisulfate (KHSO
4), the glycerol portion of the molecule is dehydrated to form the unsaturated aldehyde, acrolein (CH2=CH–CHO), which has the odor peculiar to burnt cooking grease. More modern methods exist.[8]

In the US, EPA method 603 is designed to measure acrolein in industrial and municipal wastewater streams.[19]